A high percentage of printed stock such as book covers or greetings cards needs to be creased before the next operation of folding can be carried out.
A known device for creasing stock is described in international patent application WO 00/55080. The device consists of a first drum mounted on a first rotary shaft, the first drum having at least one groove for holding a resilient ring that protrudes from the groove. A second drum is mounted on a second, parallel rotary shaft and has at least one corresponding groove. When the resilient ring protruding from the first drum is aligned with the groove of the second drum, a sheet of the stock fed between the two rotating drums will be creased by the pressure of the resilient ring deforming the paper into the groove of the second drum.
The first and second parallel shafts may conveniently be the top and bottom shafts of a conventional folding machine. The first drum is clamped on the first shaft so that the drum and the resilient ring rotate with the shaft. The second drum may be clamped on the second shaft with the corresponding grooves in alignment. Alternatively, as described in international patent application WO 2004/073966, the second drum may be mounted on bearings so that it can rotate independently of the second shaft and in a preferred arrangement can also slide axially along the second shaft.
A first problem with the aforementioned prior art is that the resilient ring must be stretched beyond its working diameter in order to pass over the outer surface of the drum and reach the groove. Despite the resilience of the ring, this stretching—particularly if done repeatedly during the lifetime of the ring—may cause the ring to lengthen so that it does not retract fully into the groove and does not have sufficient tension to grip the groove tightly. As a result, creasing may become less reliable and—because a loosely held ring is continuously deformed as the drum turns—the lifetime of the ring may be shortened.
A second problem with the aforementioned prior art is that a new ring can only be added to the creasing device by removing the shaft from the machine in which it is mounted. Although the device may provide space for storage of spare rings, their lifetime is limited and the rings will eventually need to be replaced. The difficulty of removing the shaft varies between machines but is always a time-consuming operation, during which the entire machine cannot be used for any of its functions.